test: Separated query and loading internal sleep configs

To make schema cache loading wait tests robust it is necessary to provide three separate internal config variables:
* "internal-schema-cache-query-sleep" - introduces delay in schema queries execution
* "internal-schema-cache-load-sleep" - introduces delay between schema queries execution and processing their results
* "internal-schema-cache-relationship-load-sleep" - introduces delay in processing relationship query results

Thanks to these changes it is now possible to test various schema loading scenarios with the right granularity robustly (eg. make sure requests wait for schema loading but not for relationship loading).

test/io/test_big_schema.py

@@ -2,7 +2,6 @@
 
 import pytest
 
-from util import parse_server_timings_header
 from postgrest import run
 
 
@@ -14,7 +13,7 @@ def test_requests_with_resource_embedding_wait_for_schema_cache_reload(defaulten
         "PGRST_DB_SCHEMAS": "apflora",
         "PGRST_DB_POOL": "2",
         "PGRST_DB_ANON_ROLE": "postgrest_test_anonymous",
-        "PGRST_SERVER_TIMING_ENABLED": "true",
+        "PGRST_INTERNAL_SCHEMA_CACHE_RELATIONSHIP_LOAD_SLEEP": "5100",
     }
 
     with run(env=env, wait_max_seconds=30) as postgrest:
@@ -27,10 +26,7 @@ def test_requests_with_resource_embedding_wait_for_schema_cache_reload(defaulten
         response = postgrest.session.get("/tpopmassn?select=*,tpop(*)")
         assert response.status_code == 200
 
-        plan_dur = parse_server_timings_header(response.headers["Server-Timing"])[
-            "plan"
-        ]
-        assert plan_dur > 10000.0
+        assert response.elapsed.total_seconds() > 5
 
 
 def test_requests_without_resource_embedding_wait_for_schema_cache_reload(defaultenv):
@@ -41,7 +37,8 @@ def test_requests_without_resource_embedding_wait_for_schema_cache_reload(defaul
         "PGRST_DB_SCHEMAS": "apflora",
         "PGRST_DB_POOL": "2",
         "PGRST_DB_ANON_ROLE": "postgrest_test_anonymous",
-        "PGRST_SERVER_TIMING_ENABLED": "true",
+        "PGRST_INTERNAL_SCHEMA_CACHE_LOAD_SLEEP": "1100",
+        "PGRST_INTERNAL_SCHEMA_CACHE_RELATIONSHIP_LOAD_SLEEP": "5000",
     }
 
     with run(env=env, wait_max_seconds=30) as postgrest:
@@ -54,10 +51,10 @@ def test_requests_without_resource_embedding_wait_for_schema_cache_reload(defaul
         response = postgrest.session.get("/tpopmassn")
         assert response.status_code == 200
 
-        plan_dur = parse_server_timings_header(response.headers["Server-Timing"])[
-            "plan"
-        ]
-        assert plan_dur < 10000.0
+        assert (
+            response.elapsed.total_seconds() > 1
+            and response.elapsed.total_seconds() < 5
+        )
 
 
 # TODO: This test fails now because of https://github.com/PostgREST/postgrest/pull/2122